Brushless DC motors which are driven by an inverter frequently include feedback controls for maintaining a desired phase angle relationship between the position of the rotor and the stator as each winding is energized. For example, it is frequently desired to energize a winding so as to maintain the maximum amount of torque output for the motor at any given speed. This requires that the windings be energized when the rotor is in some predetermined position with respect to the stator windings.
The torque output is at a maximum when the stator current position, as determined by its driving inverter, is at an angle of 90.degree. with respect to the magnet flux position. The magnets here are assumed to be mounted on the rotor which is directly connected to the output shaft. Thus, to maintain the torque at maximum, the current must be switched sequentially in step with magnet rotation and at specific times. However, the maximum torque position at this 90.degree. phase angle is an unstable point. If a deviation from the 90.degree. position occurs, the torque declines and there is no restoring torque to regain alignment. Therefore, some type of feedback is necessary.
Since varying the inverter switching angle from the optimum position only reduces the torque, the motor speed control must be done by another means which can both increase and reduce torque. Typically, some type of stator voltage control method is used similar to that used in a conventional DC mechanical commutator motor.
Conventionally, many DC motors of this type employ external sensors such as Hall effect sensors or inductors to measure the flux and to provide a signal to the inverter to switch to the next phase in sequence when the flux reaches a predetermined threshhold. Examples of U.S. patents that employ such external sensors can be found in LaFuze No. 4,295,085, Abraham No. 3,601,678, Pawletco No. 3,518,516 and Fertig No. 3,483,457. The problem with such external sensors is that as the motors become smaller, there is less space available for mounting.
Since the windings in such motors are switched on in sequence, there will, at times, be at least one winding which is completely unenergized. In the past, systems have been proposed in which the back emf across such unenergized windings is sensed in order to provide a switching signal for the commutator. An example of such a motor is shown in Wright U.S. Pat. No. 4,162,435. In this device, the back emf induced across an unenergized winding is sensed and this voltage is fed to an integrator. When the output of the integrator reaches a preset reference point, a signal is provided which enables the inverter to switch, energizing the next winding of the motor. The back emf is sequentially sensed on all of the motor windings in turn so that the integrated output provides the switching signals for energizing each of the windings in turn. The problem with such a system is that the switching signals for the inverter are entirely dependent upon sensing each back emf of each unenergized winding. If even one such signal is missed, the motor control becomes lost with no means of recovery. Devices that take an approach similar to that shown in Wright, are shown in the U.S. Pat. No. 4,172,050 to Nagasawa and Alley et al. U.S. Pat. No. 4,250,435. Also similar in concept are U.S. Pats. No. 4,401,934 to Dolland et al. and Dolland U.S. Pat. No. 4,394,610.
In yet another type of motor, as shown in Alley et al. U.S. Pat. No. 4,250,435, rotor position is sensed when the back emf across an unenergized winding integrated over a predetermined time period reaches a threshhold as measured in a comparator. This provides a clock signal that is compared in a phase comparator with a second clock signal that represents desired rotor position for a particular speed. Any variation in time between the two clock signals limits the amount of current supplied to a particular winding when it is energized. If the actual rotor position leads the desired rotor position, less current is supplied to the winding. More current is supplied to the winding if the actual rotor position lags the desired rotor position. By varying the current supplied to the motor, the speed of the motor can be regulated. Note that the position of the rotor with respect to the inverter switching must be controlled by another circuit. This is a relatively complex system that depends upon the presence of an error clock pulse signal each time a winding is energized to maintain the proper speed.
Still other types of systems rely upon auxiliary sensing coils to develop voltages representative of desired rotor position. As mentioned previously in connection with other devices which may use Hall effect sensors and the like, the use of auxiliary coils is not practical in very small motor systems. Examples of these types of motors include U.S. Pats. No. 4,266,432, to Schroeder et al., Fulton et al. U.S. Pat. No. 4,275,343, Fulton U.S. Pat. No. 4,455,513, D'Atre et al. U.S. Pat. No. 4,088,934 and D'Atre et al. U.S. Pat. No. 4,088,935.